1. Field of the Invention
The present invention relates to a process and a device for the vaporisation injection of samples in equipment for gas chromatographic analysis, in particular in absence of splitting of the compounds dissolved in the solvent, specially but not exclusively for samples having volumes exceeding 5 microliters and to be analyzed in capillary columns.
2. Description of the Prior Art
As it is well known, two main methods for sample injection in gas chromatographic analysis are presently used, well defined between one another and generally alternatively applied according to the analytical modes foreseen: vaporisation injection and non vaporising direct on column injection, the so-called "cold on-column injection".
In the vaporisation injection two different injection modes are in turn used, i.e. the split injection and the splitless one, namely in absence of splitting. The sample injection is generally performed by means of a syringe. The syringe needle injects the sample through an elastomeric membrane (septum) into a glass liner positioned inside the vaporisation chamber. In split injections, an even considerable percentage of the whole injected and vaporised sample is eliminated to the atmosphere; this kind of injection is used for relatively concentrated samples in order to avoid to introduce into the gas chromatographic column enormous amounts of sample; in the split injection glass liners filled with glass wool or a packing are commonly used.
When on the contrary the concentration of the compounds to be analysed is low, it is necessary to use the other injection mode, the splitless one, that allows to transfer into the column all the vapors of the components to be analysed in order to achieve the required sensitivity.
The conventional splitless injection takes place at a constant temperature, such as to allow the vaporisation of all sample components, and therefore at a temperature definitely higher than the solvent boiling point.
The vapors of the sample components, solvent included, are conveyed to the column during a preset time period called splitless time. During this period the sample compounds are quantitatively transferred to the column.
Once this time is over, the splitting valve is opened (located in the lower portion of the injector) and the excess of solvent, that has not been transferred into the column, is discharged to the atmosphere.
The splitless injection is usually performed by using empty glass liners. However, the use of glass liners with a glass wool cap was mentioned (Wylie et al.--Journal of High Resolution Chromatography 649-655--Vol. 14, October 1991). According to said publication, the glass wool "seems to reduce discrimination, but can give rise to active sites that catalyse the decomposition of labile molecules".
In order to avoid processes of discrimination of the heaviest components that take place in the conventional splitless injection, an injection technique with vaporisation at programmed temperature (PTV) has been developed.
By this technique the sample injection is carried out under cold conditions, at a starting temperature below the solvent boiling point and under a flow of carrier gas. Once the injection is performed, the injector body is heated in a programmed way to allow vaporisation and transfer to the column of the sample components.
It is thus possible to partially eliminate the solvent through a multi-step heating of the vaporiser (PTV solvent splitting technique) . To eliminate the solvent, the vaporiser is heated to an intermediate temperature near the solvent boiling point before heating it to higher temperatures.
At this intermediate temperature the solvent is eliminated thanks to the carrier gas through the splitting valve, or in backflush, through another valve (PTV solvent splitting or PTV solvent backflushing).
The PTV solvent splitting technique was used for injection of large volumes of sample (solvents) by Vogt et al. J. Chromatogr. 174 (1979) 437 and by Termonia, Lacomblez and Munari (HRC & CC 11/1988) 890.
Another PTV injection system for the injection of larger amounts of sample is based on the solvent vaporisation by "overflow" principle, in U.S. Pat. No. 3,859,209 and more recently, with different features in EP-A 461 438. In this technique the injector (of PTV type in the recent embodiment) is packed with a suitable material and heated to a starting temperature slightly above the solvent boiling point at the working pressure. The sample injection is performed in presence or in temporary absence of carrier gas and in splitless configuration.
Under said conditions the solvent vapors escape to the atmosphere while the higher boiling components of the sample remain on the packing. After completion of solvent evaporation the vaporiser temperature is increased to allow vaporisation of said components and their passage to the chromatographic column by means of the carrier gas. During this transfer the vaporiser is kept in splitless configuration.